There is an ever increasing need to remove solids from liquid streams which can contain solids. Many waste streams from industrial, manufacturing or other processes often contain solids which are undesirable or which can pollute natural waterways if such waste streams are directed into such waterways without complete solids separation.
For example, the separation of precipitated metal hydroxides from effluent water is a most cumbersome and expensive part of metal finishing waste treatment. Precipitated metal hydroxides, such as nickel, chromium, zinc, copper, iron and the like, are bound with water in effluent from plating plants and the like, and the floc of metal hydroxides formed has nearly the same specific gravity as the water. This factor tends to keep the flocculent precipitate in suspension, and it has proven most difficult and time consuming to attempt to settle the precipitated particles in settling tanks. It has been found that the more dilute the waste, the harder it is to settle the precipitated hydroxides. In general, settling tanks require an inordinate amount of time to perform their settling function to any material extent, and are incapable of removing the precipitates in sufficient quantities to satisfy present and proposed government standards with respect to effluent purity. In addition, because of the relatively lengthy time required for settling, it may take an undesirably long time to determine if the effluent from such settling tanks was completely treated to precipitate all the metal ions present as hydroxides. If not, then the effluent has to be retreated with caustic to precipitate the metal ions and then again permitted to settle the metal hydroxides formed. Additives, such as aluminum sulfate, ferric salts, or polyelectrolytes are used in settling processes in an effort to promote the precipitation and settling of the precipitates.